One Runner’s Suffering Is Another’s Inspiration

Peter Sagal, the host of the NPR show “Wait Wait... Don’t Tell Me” — who happens to be an avid runner and a columnist for Runner’s World — thinks so. Or at least that’s what he told me in a recent e-mail exchange.

“What is it about the pain of endurance sports that’s fun?” he asked me. He added that when asked why he keeps running races, “I say...um...because the pain is sort of the point? Because it’s good to push yourself to the point of breaking?

“My thesis is that the pain isn’t an obstacle to achievement so much as part of the achievement. We actually want to suffer.”

Um, no, I replied. I don’t want to suffer; I run, among other reasons, for the euphoria. My friend and running partner Jennifer Davis, whom I brought into the discussion because she runs more than anyone else I know, agreed with me and added that people endure pain for a variety of reasons.

In races, for example, many of us keep going because we want to see how well we can do. Some do it because they are stubborn. The Japanese writer Haruki Murakami, who took pride in finishing every race, wrote in his book “What I Talk About When I Talk About Running” (Knopf, 2008) that he wants his epitaph to read, “At least he never walked.”

The three of us ended the exchange with no clear agreement. Jen and I were on one side and Peter on the other. But it made me wonder whether scientists had addressed the question of pain in exercise. It turned out that they had: There is actually a body of new research that explains why Peter had one idea about pain and Jen and I had another.

The problem, it seems, is that we have only one word, “pain,” for something that should have many descriptors, as in the old legend that Eskimos have many words for snow.

Markus Amann, a muscle researcher at the University of Utah and the Salt Lake City Veterans Affairs Medical Center, says that what actually stops or slows most people during exercise is fatigue, not pain. It is regulated by a group of nerve fibers, the so-called ergoreceptors, that respond to a combination of metabolites released by muscles during exercise — calcium ions, lactate, hydrogen ions. In response, the brain “decides” to slow down, Dr. Amann explained.

But, he continued, evidence suggests that the brain makes sure to slow down or drastically limit the exercise when the muscles still have some reserve. In extreme circumstances, the signal to slow down can be overridden, allowing an athlete to tap into that muscle reserve — which is why elite marathoners can put on a burst of speed at the end of a race, only to collapse when they cross the finish line.

A second set of nerve fibers, the nociceptive nerves, signal deep muscle pain. Dr. Amann’s colleague Alan R. Light reports that the nociceptive nerves look exactly like ergoreceptive nerves but respond to much higher concentrations of the muscle metabolites — levels so high that they are not normally associated with intense exercise, perhaps because a signal to slow down or stop comes on before levels get that high. And it is not known whether these nociceptive nerves are also involved in making people stop or slow down when they are exercising.

Dr. Light and Dr. Amann showed the effects of the two types of nerve fibers by injecting muscle metabolites into the muscle at the base of subjects’ thumbs. At lower concentrations, the fatigue nerves were dominant, and subjects said their thumb muscle felt heavy, exhausted. At concentrations higher than would be expected during normal exercise, the pain nerves were activated, and people complained of aching and heat.

So fatigue, it seems, is the body’s way of telling you not to run — it’s not a way of motivating you to run. Of course, there is a certain pleasure in the fatigue after a long, hard run — that drained, depleted feeling. But many of us will attest that the depleted feeling pales in comparison with euphoria, which is unpredictable and unforgettable. And it makes sense that euphoria would be a strong motivator, I told Peter. In psychology, the most powerful way to induce a behavior is to reward it only sporadically and unpredictably. Euphoria is just such a reward: You never know when it is likely to come over you.

Peter replied: “But if you’re so fatigued that every step is an effort, and your legs are giving out, and you’re chafing yourself bloody, and the heat is making you lightheaded (this is where I was toward the end of Chicago ’07, for example), where are you on that spectrum?

“How about if you’re fighting off persistent cramps in the calves, as I was at the end of NYC ’09?” Where, he asked, “does discomfort end and pain begin?”

Maybe there is some confusion here between the pain of fatigue and the pain of an acute injury, like the kind that comes from a broken bone or a torn tendon — which, researchers say, can make you stop immediately, since you are unable to override the sensation. That is the kind of pain that Jen and I described in our e-mail exchange; Peter seemed to be referring mainly to fatigue.

That means we were assigning very different definitions to that broad term “pain.”

Dr. Amann interprets pain the way Jen and I do.

“I used to be a cross-country skier,” he said, “so when people tell me their exercise is so painful, I know exactly what they are talking about.” But, he added, “from a scientific point of view, it is not pain.”

A version of this article appears in print on May 17, 2011, on page D1 of the New York edition with the headline: One Runner’s Suffering Is Another’s Inspiration. Order Reprints|Today's Paper|Subscribe